An optical disc has reached the substantial limits of resolution of an optical system with commercialization of Blu-ray discs using a blue semiconductor laser and a high NA objective lens. It is conceivable that multilayered recording layers will become important in the future, for the purpose of further increasing the capacity. In the multilayer optical disc, since there is a need to make the detected light quantities from the respective recording layers substantially equal to each other, the reflectivity of light from a specific recording layer must be reduced. However, since there is a need to increase the capacity of the optical disc and increase the dubbing speed of a video, a data transfer rate is continuously increased, and in this state, the S/N ratio of the reproduced signal cannot be sufficiently ensured in the future. Accordingly, in order to advance the multiplayer and higher speed of the future recording layers at the same time, the higher S/N ratio of detected signals is essential.
On the other hand, as another approach for increasing the number of recording layers, a method has been studied in which, like normal optical discs such as a CD or a DVD, lights are focused on a recording medium up to a substantially diffraction limit, and two opposed lights are focused at the same position to record interference fringes (standing waves) of those two lights in the vicinity of a focused point (for example, refer to R. R. Mcleod et al., “Microholographic multilayer optical disk data storage,” Appl, Opt., Vol. 44, 2005, pp. 3197, and JP-A-2007-220206 (corresponding to EP1986187A)). In this system, while the surface recording density is comparable with that of the conventional optical disc, there is no need to provide a physical recording layer on the recording medium. This leads to such advantages that the multilayered recording layer is easily realized, multiple recording is enabled to facilitate the high capacity, and although a system that records interferences is applied, a strict tolerance such as a page data hologram is not required, and the implementation is relatively easy. Similarly, in this system, the quantity of reflected light from the recorded interference fringes is generally very weak as compared with the conventional optical disc, and resultantly the higher S/N ratio of the detected signal is essential.
The techniques related to the higher S/N ratio of the reproduced signal of the optical disc are disclosed in, for example, JP-A-Hei5 (1993)-342678, JP-A-Hei6(1994)-223433, and JP-A-Hei6(1994)-068470. JP-A-Hei5(1993)-342678 and JP-A-Hei6(1994)-223433 aim at the higher S/N ratio of the reproduced signal of a magnetooptic disc. In the technique, a light from a semiconductor laser is split before being irradiated onto an optical disc, and one split light that is not irradiated onto the optical disc is coupled, with another light reflected from the optical disc to interfere with each other. The amplitude of a weak signal is then amplified by increasing the quantity of light that is not irradiated onto the optical disc. In the differential detection of a transmitted light and the reflected light of a polarizing beam splitter conventionally used in the signal detection of the magnetooptical disc, an original incident polarization component is interfered with a polarization component orthogonal to an incident polarization direction, which is developed by polarization rotation due to the magnetooptical disc. Then, the orthogonal polarization component is amplified by the incident polarization and detected. Accordingly, although the signal can be increased by increasing the original incident polarization component, there is a need to suppress the light intensity incident to the optical disc to some degree or lower so as not to erase or overwrite the data.
On the contrary, in the above conventional technique, a signal light is separated from a light to be interfered in advance, and the light is interfered with the signal light without being focused on the disc. Then, the intensity of light to be interfered for signal amplification can be increased regardless of the light intensity of the disc surface. As a result, in principle, the S/N ratio compared with the noise of an amplifier that converts a photocurrent from a photodetector into a voltage can be enhanced more as the intensity is higher in an allowable range of the light intensity.
JP-A-Hei6(1994)-068470 aims at the higher S/N ratio of the reproduced signal of the optical disc using a photochromic medium in which, like JP-A-Hei5(1993)-342678 and JP-A-Hei6(1994)-223433, a light not irradiated onto the optical disc is interfered with a reflected light from the optical disc to amplify the signal. Likewise, in the optical disc using the photochromic medium, the deterioration of the medium is accelerated more as the incident light intensity is higher because of signal reproduction. Therefore, like the above magnetooptical disc, the intensity of light to be irradiated onto the recording medium is limited.
In JP-A-Hei5(1993)-342678, two lights are interfered with each other to detect the interference light intensity. In this case, an optical path length of the disc reflected light to be interfered is made variable to ensure the interference signal amplitude. In JP-A-Hei6(1994)-223433, JP-A-Hei6(1994)-068470, and JP-A-2007-317284, differential detection is also conducted in addition to the interference light intensity detection. As a result, the intensity components of the respective lights that do not contribute to the signal are canceled, and the signal amplitude is doubled to provide the higher S/N ratio.
In general, the interference signal obtained by interference of two lights depends on a phase difference (optical path length) between two lights to be interfered. On the contrary, in JP-A-Hei5(1993)-342678, a triangular prism that has been inserted into an optical path is made movable in the incident optical axis direction to stabilize an optical path length difference. Likewise, in JP-A-2007-317284, an entire interference optical system is made to follow the optical disc to cancel the optical path length difference caused by surface wobbling accompanied by rotation of the optical disc. Also, a position of a mirror that reflects the light that is not applied to the optical disc is made movable in the optical axis direction to stabilize the optical path length difference. In JP-A-2008-65961, plural interference signals different in interference state from each other are generated, and a signal is generated by arithmetic operation of those interference signals to output an amplified signal not depending on the phase difference.